S-transformation Technique In Three-dimensional Telecentric Measurement

With the development of industries, manufactures and defect inspection coming into refinement and intelligence, there is a need to study a dynamic and highly precise three-dimensional telecentric measurement method, which is aimed at measuring micro-components. In lots of measurement methods, because optical measurement only need to actively illuminate the object to be tested, and capture the light signal that modulated by the object, then the object's 3D shape can be gained. These kinds of optical measurement have many characteristics such as simple principle, convenient measuring access, low cost and non-contacting. And the optical measurements based on projecting sinusoidal fringe patterns, that are widely used nowadays, can get the object's depth taking advantage of the phase and triangle method. In all of these phase defecting methods, Fourier profirometry can separate the base spectrum which includes information about object's depth through filtering. However, once the slope of the depth changes largely, or the noise and nonlinearity of the picture are large, the spectrum will appear spectrum aliasing and lead to seriously phase getting error. What's more, the S transform proposed by R. G. Stockwell et al based upon Fourier profirometry, can avoid spectrum aliasing effectively, hence this essay is mainly on the S transform in telecentric measurement. And the main studying contents are as follows:1. On the basis of deeply understanding the S transform, this paper discusses the two accesses of getting the phase of fringes for S transform, and analyzes the vital factors that influent the phase accuracy of the two ways. Given this, the paper considers the time-frequency distribution when the generalized S transform matrix is disturbed by the noise and nonlinearity of the system, designs a new asymmetric filter to use for the filtering phase getting method of generalized S transform. The experiments show that this kind of new filter can restrain the noise and nonlinearity of the system well, and largely improve the phase getting accuracy when using the generalized S transform filtering method.2. Based on the wrapped phase map of single sinusoidal fringes above, this article studies the methods of getting absolute phase map through one wrapped phase matrix, and the reasons of the propagation of phase unwrapping error. Consider this, the paper improves the traditional line-by-line phase unwrapping method, and proposes utilizing the S transform ridge to be the phase unwrapping path, and combining the quality map and branch-cut to get the absolute phase. This can not only effectively suppress the error propagation but the speed of the progress of the phase unwrapping.3. This paper, combining the telecentric imaging system that is setup by our research group, deduces the linear relationship between the phase and depth, and the Scale orthogonal projection model in the telecentric system, then restore the X, Y, Z world coordinates of the object to be measured through the phase solving results of the generalized S transform and the system parameters.At last, this paper uses a highly precise ceramic calibration target and a electronic elevator stage to calibrate the system, and images the real object's 3D shape based on the proposed generalized S transform phase getting method. The experiments verifies the validity and effectiveness of the method proposed.